Secondary (i.e., rechargeable) lithium ion batteries have been implemented as a power source into a wide variety of stationary and portable applications. Their structure and electrochemical reaction mechanism provide them with several desirable characteristics including a relatively high energy density, a relatively low internal resistance, a general non-appearance of any memory effect when compared to other types of rechargeable batteries, for example, nickel-cadmium batteries, and a low self-discharge rate. These characteristics have made lithium ion batteries the preferred mobile power source for portable consumer electronics such as laptop computers and cell phones. Larger-scale versions that interact with a multitude of interconnected systems have also been designed and manufactured by the automotive industry in an effort to improve vehicle fuel efficiency and reduce atmospheric pollution. The powertrains of hybrid electric vehicles (HEV) and extended range electric vehicles (EREV), for example, rely on the cooperative effort of lithium ion batteries and a hydrocarbon-fueled internal combustion engine to generate torque for vehicle propulsion.
A lithium ion battery generally contains one or more individual electrochemical battery cells that include a negative electrode, a positive electrode, and a porous polymeric separator sandwiched between the confronting inner face surfaces of the electrodes under a compressive force. The negative electrode generally includes a lithium host material that stores intercalated lithium at a relatively low electrochemical potential (relative to a lithium metal reference electrode). The positive electrode generally includes a lithium-based active material that stores intercalated lithium at a higher electrochemical potential than the lithium host material (relative to the same lithium metal reference electrode). The interadjacent porous separator includes opposed major surfaces that intimately contact the confronting inner face surfaces of the electrodes, and has typically been composed of a polyolefin such as polyethylene and/or polypropylene. A main function of the separator is to provide a porous and electrically insulative mechanical support barrier between the negative and positive electrodes. Each of the negative electrode, the positive electrode, and the separator is wetted with a liquid electrolyte solution that can communicate lithium ions. The liquid electrolyte solution is typically a lithium salt dissolved in a non-aqueous liquid solvent.
An interruptible external circuit electrically connects the negative electrode and the positive electrode to provide an electrical current path around the separator to electrochemically balance the migration of lithium ions. Metallic current collectors intimately associated with each electrode supply and distribute electrons to and from the external circuit depending on the operating state of the electrochemical battery cell. The external circuit can be coupled to an electrical load (during discharge) or an applied voltage from an external power source (during charging) through conventional electronic connectors and related circuitry. A voltage of approximately 2.5V to 4.3V is usually attained in each electrochemical battery cell during battery discharge. Greater overall battery power levels can be achieved, if necessary, by linking together a suitable number of similar electrochemical battery cells with their negative and positive electrodes connected in series or in parallel to corresponding common terminals. Current lithium ion batteries intended to be used in a vehicle powertrain typically include anywhere from 10 to 150 individual electrochemical battery cells. Several of these lithium ion batteries can be further connected in series or in parallel and packaged together to form a lithium ion battery pack that achieves a desired overall voltage and current capacity.
The individual electrochemical battery cell of a lithium ion battery operates by reversibly transporting lithium ions between the negative electrode and the positive electrode. The liquid electrolyte solution facilitates transport of the lithium ions through the separator. The flow direction of the lithium ions depends on whether the electrochemical batter cell is operating in a discharge state or a charge state. The lithium ions migrate from the negative electrode to the positive electrode during discharge and vice-versa during charging. The flow direction of the electrons through the external circuit mimics that of the lithium ions.
The discharge phase of the electrochemical battery cell can proceed when the negative electrode contains a sufficiently high concentration of intercalated lithium while the positive electrode is sufficiently depleted. The interruptible external circuit, when closed, provokes extraction of the intercalated lithium from the negative electrode. The extracted lithium splits into lithium ions and electrons. The lithium ions dissolve into the liquid electrolyte solution and migrate through the separator towards the positive electrode where they intercalate into the lithium-based active material. The electrons flow through the external circuit from the negative electrode to the positive electrode (with the help of the metallic current collectors) to balance these half-reactions. The flow of electrons through the external circuit can be harnessed and fed through the external load until the concentration of intercalated lithium in the negative electrode falls below a minimum effective level or the external circuit is opened.
The charge phase of the electrochemical battery cell can proceed after a partial or full reduction of its available capacity (through the discharge phase). To charge or re-power the cell, the external circuit is subjected to an applied voltage that originates from an external power source and which is sufficient in magnitude to accomplish charging in a reasonable time frame. The applied voltage drives the reverse of the discharge phase electrochemical half reactions; that is, during charging, intercalated lithium is extracted from the positive electrode to produce lithium ions and electrons. The lithium ions are carried back to the negative electrode through the separator and the electrons are driven back to the negative electrode through the external circuit. The lithium ions and the electrons reunite and replenish the negative electrode with intercalated lithium for the next battery discharge phase. Many thousands of substantially full-power discharge/charge cycles can be accomplished over the practical lifetime of the electrochemical battery cell.
The lifetime and performance of the electrochemical battery cell can be adversely impacted by a wide variety of expected and unforeseen factors. Exposure of the electrochemical battery cell to temperatures of 100° C. and above can cause the polyolefin separator to shrink, soften, and even melt if the temperature approaches 130° C. Such high temperatures can be attributed to charging-phase heat generation, ambient atmospheric temperature, or some other source. The temperature-initiated physical distortion of the polyolefin separator may ultimately permit direct electrical contact between the negative and positive electrodes and cause the electrochemical cell to short-circuit. Battery thermal runaway is also a possibility if the electrodes come into direct electrical contact with one another to an appreciable extent. A separator that can function reliably at the high-temperatures possibly encountered in an electrochemical battery cell of a lithium ion battery without affecting lithium ion movement is therefore needed.